Multipod Bi(Cu2-xS)n Nanocrystals formed by Dynamic Cation–Ligand Complexation and Their Use as Anodes for Potassium-Ion Batteries

We report the formation of an intermediate lamellar Cu–thiolate complex, and tuning its relative stability using alkylphosphonic acids are crucial to enabling controlled heteronucleation to form Bi(Cu2-xS)n heterostructures with a tunable number of Cu2-xS stems on a Bi core. The denticity of the phosphonic acid group, concentration, and chain length of alkylphosphonic acids are critical factors determining the stability of the Cu–thiolate complex. Increasing the stability of the Cu–thiolate results in single Cu2-xS stem formation, and decreased stability of the Cu–thiolate complex increases the degree of heteronucleation to form multiple Cu2-xS stems on the Bi core. Spatially separated multiple Cu2-xS stems transform into a support network to hold a fragmented Bi core when used as an anode in a K-ion battery, leading to a more stable cycling performance showing a specific capacity of ∼170 mAh·g–1 after 200 cycles compared to ∼111 mAh·g–1 for Bi–Cu2-xS single-stem heterostructures.


Bi-Cu 2-x S nanocrystal (NC) synthesis.
In a typical synthesis, 130.9 mg (0.5 mmol) Cu(acac) 2 , 157.7 mg (0.5 mmol) BiCl 3 , 0.1-0.5 mmol of alkyl phosphonic acid/ diphosphonic, were mixed with 2 ml OLA and 8 ml ODE solvent mixture in a 3-neck round bottom flask (RBF) and reaction mixture was evacuated at 105 °C for 25 min (5 min ramp to 105 °C and 25 min soak). The vacuum pressure was kept below 200 mTorr during evacuation. Afterwards, the reaction mixture was heated to 160 °C under an argon atmosphere (3 min ramp to 160 °C). 1ml of thiol mixture (0.875 ml 1-DDT and 0.125 ml t-DDT) was injected when the temperature reached 135 °C. After thiol injection the reaction mixture turned to bright orange from blue and finally turned to black above 147 °C ( Figure S1). When the temperature of reaction vessel reached 160 °C it was allowed to proceed for another 5 min of growth time. Afterwards the heating mantle was removed to terminate the reaction by natural cooling till 80 °C. Upon reaching 80 °C, 8 ml of Ethylacetate was injected to quench the reaction. For all experiments 0.1 mmol of alkyl phosphonic acid is used unless mentioned otherwise.
Supporting Information Figure S1. The reaction mixture color at different temperatures.
1.3. NC purification procedure. The heterostructures synthesized and quenched with 8 ml of Ethylacetate were poured into a 50 ml centrifuge tube and vortexed well. After that, the NC solution was centrifuged at 5000 rpm for 5 min. The pellet was collected and dispersed in 10 ml of Tol, and 10 ml of Ethylacetate was further added and vortexed to disperse the NCs well. The NC solution was again centrifuged at 5000 rpm for 5 min and the process was repeated another 2 times and dried at 80 °C overnight in vacuum.

Aliquot study.
During NC growth 1 ml solution from the RBF was withdrawn at desired temperature and time after thiol injection. To ensure minimal depletion in precursor concentration a maximum of 2 ml of reaction solution in total was withdrawn from RBF.
After withdrawal, the growth was immediately quenched by ejecting into 2 ml of Tol. The NCs in 2ml of Tol were dispersed in 2ml of IPA and centrifuged for 5 min at 5000 rpm.
Followed by another two cycles of redispersion in 2 ml Tol and 2ml IPA and centrifugation at 5000 rpm for 3 min. For isolating the Cu-thiolate complex the aliquot collected at 140 °C is centrifuged at 8000 rpm. The supernatant is separated and mixed with 5 ml of methanol to isolate the Cu-thiolate as precipitate and dried to obtain an orange color powder and stored in glovebox for further characterization ( Figure S12). For NMR characterization the aliquot collected at 140 °C was thermally quenched and no toluene was added. After cooling it down to room temperature the liquid portion of the aliquot characterized through 1 H NMR and 31 P NMR (JEOL 400 MHz NMR spectrometer) in CDCl 3 . The peaks were referenced to the residual chloroform peak at 7.26 ppm for 1 H NMR.     C.E (%) Figure S9. Comparison of cycling performances of singlepod (SP) and multipod (MP)-based anodes at different voltage ranges.

Post characterization of Bi-Cu 2-x S-based anodes
Figure S10. TEM image (inset) and SAED pattern of Multipod (MP) based electrode (left), and single pod (SP) based electrode (right) after 50 cycles in discharged state (0.01 to 1.5 V vs K/K + ).